1. Field of the Invention
This invention relates to a resistor network the resistance of which can be adjusted to suit a particular requirement. In its preferred aspects, this invention relates to a variable resistor network that can be programmed to provide an accurate simulation of a resistive sensor or another variable resistor, preferably adjustable over a large resistance range but with a fine control of total resistance.
2. Description of the Related Art
Many control systems use variable resistance devices as sensors to indicate the value of a parameter that is important to the operation of the system. Examples vary from simple fuel gauges that indicate the remaining amount of fuel in a tank to sensors that measure the temperature of air at an intake or an exhaust, water temperature or mechanical stress. In test applications it is important to be able accurately to simulate these variable resistance devices to check that the control system responds correctly by adjusting the operation of the system or by providing appropriate warning indicators. The behaviour has to be tested under both normal conditions and under fault conditions; for example, when a sensor fails or should the interconnection system fail, the readings from the sensor will be distorted.
Modern systems that control, for example, the operation of an engine can have many sensors present and the testing can be complex as engineers try to explore both the fault tolerance of the system and its behaviour as the sensors indicate changing conditions. It is not practical routinely to test the control system using a real engine, even when conducting type approval testing rather than a manufacturing test. As a result the sensors are often simulated by programmable resistors. Many of the currently available programmable resistors have significant short comings that make them less than ideal for this purpose.
There are many other applications for programmable resistors in electronic systems where there are similar requirements for a simple device that can simulate a resistor to a good precision, in a compact form factor. The most common method for implementing a variable resistor to simulate a sensor for use in an application as has been described above is to use a binary chain of resistors each of which can be shorted by a relay the contacts of which can be opened or closed under program control. An example of such a resistor is shown in FIG. 1 where each of the resistors R1, R2 . . . R16 has a reed relay arranged across its terminals and so can be taken out of circuit or included in the circuit between the resistor terminals T1 and T2.
An offset resistor (R OFF) is used to define the minimum value to which the variable resistor can be set. A binary weighted chain of resistors then provides the incremental values of resistance. The smallest resistor defines the smallest step size that can be set and the other resistors are binary weighted (1, 2, 4, 8 etc). In the example shown R1 has the smallest value of 1 ohm so the set resistance value can be nominally changed in 1 ohm steps by closing or opening the relay contacts associated with R1. For example, zero additional ohm to R OFF is obtained with all relays closed and 1 ohm by opening just the first relay. 2 ohm is obtained by opening just the second relay and 3 ohm by opening both the first and second relays.
The method is simple and efficient to implement but has some significant shortcomings:                The relays do not provide perfect shorts so the change in value is not precisely as expected. If the relay contact resistance is 0.1 ohm (which is typical for a reed relay), then the 1 ohm increments are not precisely 1 ohm. As more relays are switched in and out, the errors accumulate and a fine adjustment of 1 ohm becomes less predictable.        Resistor tolerance is a problem. When many resistors are switched out and substituted by a single resistor, the resistor tolerances can cause the total resistance of the chain to change by an amount significantly different from the 1 ohm, expected in this example.        Resistors having special values and high tolerance are hard to purchase. They are not components that are commonly available from suppliers because they are not preferred values (usually referred to as E series values), so they have to be purchased from specialist suppliers at a high cost with long lead times and large minimum order quantities.        If the application requires fine control of the resistance value, the sources of error tend to accumulate and be less predictable as the number of relays in the system increases.        Interconnection resistances of PCB tracks between the relays can produce significant errors that vary unpredictably when the required resistance value is changed.        